Chemical conversion coated plated steel sheet and method for producing same

ABSTRACT

Disclosed is a chemical conversion coated Al-plated steel sheet which has excellent weather resistance, water resistance and coating film adhesion. Specifically, a chemical conversion coating liquid is applied over and dried on the surface of an Al alloy-plated steel sheet, thereby forming a chemical conversion coating film that has a film thickness of 0.5-10 μm. The chemical conversion coating liquid contains: a fluorine-containing olefin resin, which contains 7-20% by mass of F atoms and 0.05-5% by mass of hydrophilic functional groups that are selected from among a carboxyl group, a sulfonic acid group and salts of carboxyl group and sulfonic acid group and has a number average molecular weight within the range of 1,000-2,000,000; and an oxygen acid salt, fluoride, hydroxide, organic acid salt, carbonate or peroxygenated salt of a group 4A metal.

TECHNICAL FIELD

The present disclosure relates to a chemically treated aluminum-basedplated steel sheet which demonstrates excellent weather resistance,water resistance and film adhesion, and a method of preparing the same.

BACKGROUND ART

For some plated steel sheets, a chemical conversion film is formed thatcontains an organic resin in order to prevent galling at the time offormation (e.g., see Patent Literatures 1 and 2; hereinafter referred toas “PTL 1” and “PTL 2”). PTL 1 and 2 disclose that a chemical conversionfilm which contains an organic resin such as a urethane resin is formedon a surface of a zinc-based plated steel sheet. Coating the surface ofthe plated steel sheet in such a manner with a chemical conversion filmthat contains an organic resin not only allows for galling resistance tobe improved, but also allows for corrosion resistance, tarnishresistance and/or the like to be improved.

On the other hand, a fluororesins, which exhibit excellent weatherresistance, are sometimes used as the organic resin constituting thechemical conversion film in order to improve the weather resistance ofthe chemically treated plated steel sheet. When using fluororesins forimproving weather resistance in such a manner, organic solvent-basedfluororesin compositions are frequently used. However, such organicsolvent-based fluororesin compositions have the drawbacks of being firehazards, toxicity hazards, an air pollutants, and/or the like.

Moreover, various water-based fluororesin compositions are also provided(e.g., see Patent Literature 3; hereinafter referred to as “PTL 3”).However, such water-based fluororesin compositions all require baking athigh temperatures (e.g., 180-230° C.; see PTL 3). Such ahigh-temperature baking is not feasible at painting sites (where air-dryresins are typically used), and is disadvantageous even in productionline where heat-drying is mainly employed upon formation of coatings.

In addition, in order to solve the problem pertinent in water-basedfluororesins, a curable moiety (organic functional group) is introduced,and a water-based fluororesin compositions are thereby provided fromwhich a film can be formed even at low temperatures (e.g., see PatentLiterature 4; hereinafter referred to as “PTL 4”). However, in the caseof a cured film in which organic functional group have been reacted,weather resistance is preferentially deteriorated from the curedportions, and therefore the film becomes porous and water resistancedecreases. Moreover, even when a surface treatment has been implementedusing an epoxy resin, urethane resin or the like in order to improveadhesion, the organic resin preferentially deteriorated weatherresistance, and dramatically reduces film adhesion.

CITATION LIST Patent Literature

PTL 1

-   Japanese Patent Application Laid-Open No. 2005-15834    PTL 2-   Japanese Patent Application Laid-Open No. 2005-206764    PTL 3-   Japanese Patent Application Laid-Open No. 57-38845    PTL 4-   Japanese Patent Application Laid-Open No. 05-202260

SUMMARY OF INVENTION Technical Problem

As previously described, forming a chemical conversion film containingan organic resin on a surface of a plated steel sheet allows for gallingresistance, corrosion resistance and adhesion of a film (coating) formedon the chemical conversion film to be improved. However, weatherresistance may not be sufficient when a conventional chemically treatedplated steel sheet which includes a chemical conversion film containingan organic resin is used as an exterior building material. Specifically,because many organic resins such as urethane resins are degraded byexposure to ultraviolet light, when a conventional chemically treatedplated steel sheet is used as an exterior building material, a loss in achemical conversion film that coats a surface of the plated steel sheetmay occur over time. Accordingly, when the chemical conversion film islost, a change in color, rust and/or the like may occur, the aestheticappearance may thus be damaged, and the chemically treated plated steelsheet is undesirable as an exterior building material.

As a means for improving the weather resistance of the chemicallytreated plated steel sheet, fluororesins which are superior in weatherresistance, are considered to be used as the organic resin constitutingthe chemical conversion film. The present inventors performedpreliminary experiments in which a chemical conversion film was formedon a surface of a plated steel sheet using a water-based fluororesinemulsion, which is easy to handle. As a result, ultraviolet lightresistance was improved by using the water-based fluororesin emulsion,but a film forming property, water resistance and film adhesion weredecreased. As a result of additional studies by the present inventors, areduction in these properties is assumed to be from an emulsifier thatis used to prepare the water-based fluororesin emulsion (e.g., ammoniumperfluorooctanoate) remaining in the chemical conversion film (see thebelow-described Reference Experiments).

As previously described, a conventional chemically treated plated steelsheet that includes a chemical conversion film containing an organicresin may demonstrate insufficient weather resistance. Moreover,although the weather resistance (ultraviolet light resistance) of achemically treated plated steel sheet can be improved by using awater-based fluororesin as an organic resin, because the film formingproperty, water resistance and film adhesion were reduced, weatherresistance, water resistance and film adhesion could not all beachieved.

An object of the present invention is to provide a chemically treatedaluminum-based plated steel sheet having a chemical conversion filmcontaining an organic resin, which is superior in weather resistance,water resistance, blackening resistance and film adhesion.

Solution to Problem

The present inventors found that weather resistance, water resistanceand film adhesion of a chemical conversion film were improved by usingas an organic resin a high molecular weight fluororesins in which ahydrophilic functional group was introduced and by cross-linking chainsof the fluororesin with a group 4A metal compound. Thus, the presentinventors completed the present disclosure after further testing wasperformed.

Specifically, a first aspect of the present invention relates to achemically treated aluminum-based plated steel sheet describedhereinafter.

[1] A chemically treated aluminum-based plated steel sheet, comprising:an aluminum-based alloy plated steel sheet, the aluminum-based alloycontaining 85-99% by weight of aluminum; and a chemical conversion filmhaving a film thickness of 0.5-10 μm formed on a surface of thealuminum-based alloy plated steel sheet, wherein the chemical conversionfilm is a fluororesin containing 0.05-5% by weight of a hydrophilicfunctional group selected from the group consisting of a carboxyl groupand a sulfonic acid group and 7-20% by weight of a fluorine atom, and0.1-5% by weight in terms of a metal with respect to the fluororesin ofa group 4A metal compound.

[2] The chemically treated aluminum-based plated steel sheet accordingto [1], wherein a ratio between the carboxyl group and the sulfonic acidgroup included in the fluororesin in terms of a carboxyl group/sulfonicacid group molar ratio is 5-60.

[3] The chemically treated aluminum-based plated steel sheet accordingto [1] or [2], wherein the chemical conversion film further contains aphosphate, and an amount of the phosphate with respect to thefluororesin is within a range of 0.05-3% by weight in terms ofphosphorus.

[4] The chemically treated aluminum-based plated steel sheet accordingto any of [1] to [3], wherein the chemical conversion film furthercomprises a silane coupling agent, wherein an amount of the silanecoupling agent with respect to the fluororesin is within a range of0.5-5% by weight.

[5] The chemically treated aluminum-based plated steel sheet accordingto any of [1] to [4], wherein the group 4A metal is selected from thegroup consisting of Ti, Zr, Hf and any combination thereof.

[6] The chemically treated aluminum-based plated steel sheet accordingto any of [1] to [5], further comprising a preliminary chemicalconversion film containing an oxide or hydroxide of a valve metal and afluoride of a valve metal, the preliminary chemical conversion filmformed between the aluminum-based alloy plated steel sheet and thechemical conversion film.

A second aspect of the present invention relates to a method ofpreparing a chemically treated aluminum-based plated steel sheetdescribed hereinafter.

[7] A method of preparing a chemically treated aluminum-based platedsteel sheet, the method comprising: preparing an aluminum-based alloyplated steel sheet, the aluminum-based alloy containing 85-99% by ofweight aluminum; and forming a chemical conversion film that has athickness of 0.5-10 μm, by coating and drying a chemical treatmentsolution on a surface of the aluminum-based alloy plated steel sheet,wherein: the chemical treatment solution contains a fluororesincontaining 0.05-5% by weight of a hydrophilic functional group selectedfrom the group consisting of a carboxyl group and a sulfonic acid group,7-20% by weight of a fluorine atom, the fluororesin in which anumber-average molecular weight is 1,000-2,000,000, and an oxoate, afluoride, a hydroxide, an organic salt, a carbonate or a peroxygenatedsalt of a group 4A metal; and an amount of the oxoate, the fluoride, thehydroxide, the organic salt, the carbonate or the peroxygenated salt ofthe group 4A metal is within a range of 0.1-5% by weight in terms ofmetal with respect to the fluororesin.

[8] The method of preparing the chemically treated aluminum-based platedsteel sheet according to [7], wherein a ratio between the carboxyl groupand the sulfonic acid group contained in the fluororesin in terms of acarboxyl group/sulfonic acid group molar ratio is 5-60.

[9] The method of preparing the chemically treated aluminum-based platedsteel sheet according to [7] or [8], wherein the chemical treatmentsolution further contains a phosphate, and an amount of the phosphatewith respect to the fluororesin is within a range of 0.05-3% by weightin terms of phosphorus.

[10] The method of preparing the chemically treated aluminum-basedplated steel sheet according to any of [7] to [9], wherein the chemicaltreatment solution further contains a silane coupling agent, and anamount of the silane coupling agent with respect to the fluororesin iswithin a range of 0.5-5% by weight.

[11] The method of preparing the chemically treated aluminum-basedplated steel sheet according to any of [7] to [10], wherein the group 4Ametal is selected from the group consisting of Ti, Zr, Hf and anycombination thereof.

[12] The method of preparing the chemically treated aluminum-basedplated steel sheet according to any of [7] to [11], further comprisingforming a preliminary chemical conversion film by coating and drying apreliminary chemical treatment solution on a surface of thealuminum-based alloy plated steel sheet before forming the chemicalconversion film, wherein the preliminary chemical treatment solutioncontains a valve metal salt and a fluoride ion.

Advantageous Effects of Invention

According to the present invention, a chemically treated aluminum-basedplated steel sheet may be provided, in which weather resistance, waterresistance and film adhesion are superior. Because the chemicallytreated aluminum-based plated steel sheet of the present invention issuperior in weather resistance, water resistance, corrosion resistanceand tarnish resistance, the chemically treated aluminum-based platedsteel sheet is useful as a plated steel sheet for an exterior buildingmaterial and/or the like.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing a relationship between an amount of a group 4Ametal in a fluororesin film and moisture permeability of the fluororesinfilm; and

FIG. 2 is a graph showing a relationship between a concentration of anemulsifier in a fluororesin emulsion and moisture permeability of afluororesin film.

DESCRIPTION OF EMBODIMENTS

1. Chemically Treated Aluminum-Based Plated Steel Sheet

A chemically treated aluminum-based plated steel sheet of the presentinvention includes an aluminum-based alloy plated steel sheet (originalsheet for chemical treatment) and a chemical conversion film formed on asurface of the aluminum-based alloy plated steel sheet. The chemicallytreated aluminum-based plated steel sheet of the present inventionincludes a chemical conversion film containing a high molecular weightfluororesin having a hydrophilic functional group (carboxyl group, asulfonic acid group, and/or the like) introduced thereinto and a group4A metal compound.

Hereinafter, each constituent element of the chemically treatedaluminum-based plated steel sheet of the present invention will bedescribed.

Original Sheet for Chemical Treatment

An aluminum-based plated steel sheet with excellent corrosion resistanceand aesthetic quality is used as the original sheet for chemicaltreatment. The term “aluminum-based alloy plated steel sheet” as usedherein refers to a steel plate including an aluminum-based alloy layercontaining 85-99% by weight of aluminum and a residue that issubstantially Si. An example of the aluminum-based coated alloy steelsheet includes hot-dip aluminum-9% Si plated steel sheet.

A low carbon steel, a medium carbon steel, a high carbon steel, an alloysteel and the like, may be used as a substrate steel of thealuminum-based alloy plated steel sheet. When formability is required, asteel sheet for deep drawing such as a low carbon steel alloyed withtitanium or a low carbon steel alloyed with niobium are preferable as asubstrate steel.

Preliminary Chemical Conversion Film

A preliminary chemical conversion film which contains an oxide orhydroxide of a valve metal and a fluoride of a valve metal may be formedon the surface of the aluminum-based alloy plated steel sheet that isused as the original sheet for chemical treatment. Accordingly, when thepreliminary chemical conversion film is formed on the surface of thealuminum-based alloy plated steel sheet, the chemical conversion film isformed on the surface of the aluminum-based alloy plated steel sheet viathe preliminary chemical conversion film.

The oxide or hydroxide of a valve metal demonstrates high-insulatingresistance. Accordingly, the preliminary chemical conversion film thatcontains an oxide or hydroxide of a valve metal acts as a resistivematerial against the transfer of electrons. Consequently, the reductionof dissolved oxygen contained in the moisture in air is limited, and theoxidation, which is the counter-reaction of reduction, of thealuminum-based alloy plated steel sheets can also be limited. As aresult, elution (corrosion) of a metal component from the aluminum-basedalloy plated steel sheet that forms a substrate is limited. Among these,a tetravalent compound of a group 4A element such as Ti, Zr and Hf is astable compound, and thus forms an excellent high-insulating film.

Moreover, film defects inevitably occur in the chemical conversion filmduring chemical treatment or formation. Because the substrate is exposedin a defective portion of the film, a corrosion inhibition effect cannotbe expected even if a chemically treatment is performed. On the otherhand, the above-described preliminary chemical conversion film containsa fluoride of a valve metal, and thus has a self-repair ability.Specifically, the fluoride of the valve metal is reprecipitated as apoorly-soluble oxide or hydroxide on a surface of the substrate steelthat is exposed from a film defect portion after the fluoride of thevalve metal is dissolved in the moisture contained in air. As a result,because the defective portion of the film is filled in, a self-repairability is generated.

The chemically treated aluminum-based alloy plated steel sheet of thepresent invention is prepared by performing a chemical treatment on anon-treated aluminum-based alloy plated steel sheet. Non-treatedaluminum-based alloy plated steel sheets are, however, insufficient incorrosion resistance. Accordingly, corrosion may occur during thestorage, transportation or formation of the non-treated aluminum-basedalloy plated steel sheet. Therefore, corrosion can be prevented fromactually occurring on the non-treated aluminum-based alloy plated steelsheet by forming the preliminary chemical conversion film on thenon-treated aluminum-based alloy plated steel sheet.

During the storage, transportation or formation (including welding) ofthe aluminum-based alloy plated steel sheet that includes thepreliminary chemical conversion film, a portion of the preliminarychemical conversion film may peel off, break off, or fall off.Accordingly, the surface of the aluminum-based alloy plated steel sheetis exposed, and the chemical conversion film that contains thefluororesin and the group 4A metal compound is in direct contact withthe exposed surface.

The preliminary chemical conversion film demonstrates an effect ofimproving the adhesion of the aluminum-based alloy plated steel sheetand the chemical conversion film. Accordingly, a chemical conversionfilm that is formed at a location where the preliminary chemicalconversion film has been peeled off is generally thought to exhibit areduction in film adhesion. However, aluminum is eluted from the platinglayer that is in direct contact with the chemical conversion film. Thus,the corrosion resistance and the film adhesion of the chemicalconversion film are improved by means of aluminum eluted in the chemicalconversion film. Accordingly, the adhesion between the chemicalconversion film and the aluminum-based alloy plated steel sheetincreases, and therefore, the corrosion resistance of the chemicalconversion film increases even at the location where the preliminarychemical conversion film has been peeled off. Hereinafter, a mechanismwill be described by which the corrosion resistance and the filmadhesion of the chemical conversion film are improved owing to thepresence of aluminum in the chemical conversion film.

The preliminary chemical conversion film may be formed by drying thecoating of preliminary chemical treatment solution formed on the surfaceof the aluminum-based alloy plated steel sheet as the substrate. A valvemetal salt, a fluoride ion, and water, which is a solvent, are added inthe preliminary chemical treatment solution. The valve metal salt formsa valve metal oxide, hydroxide or fluoride included in the preliminarychemical conversion film by drying the coating of the preliminarychemical treatment solution.

Examples of the valve metal may include Ti, Zr, Hf, V, Nb, Ta, Mo and W.The valve metal that is added to the preliminary chemical treatmentsolution may be a valve metal halide or oxoate, or the like. When thevalve metal salt that is added is a fluoride, the valve metal may alsoact as a fluoride ion source.

Examples of titanium salts may include K_(n)TiF₆ (where K is an alkalimetal or an alkali earth metal; n is 1 or 2), K₂[TiO(COO)₂], (NH₄)₂TiF₆,TiCl₄, TiOSO₄, Ti(SO₄)₂, and Ti(OH)₄. On the other hand, the fluorideion source included in the preliminary chemical treatment solution maybe a valve metal salt including a fluorine atom or may also be a solublefluoride (e.g., (NH₄)F).

In order to stabilize a valve metal salt in the preliminary chemicaltreatment solution, an organic acid that has a chelating action ispreferably added. The organic acid is able to stabilize the chemicaltreatment solution by chelation of metal ions. Accordingly, the additiveamount of the organic acid is set so that the molar ratio oforganic/metal ions is at least 0.02. Examples of the organic acid mayinclude tartaric acid, tannic acid, citric acid, oxalic acid, malonicacid, lactic acid, acetic acid, and ascorbic acid. Among these,oxycarboxylic acids such as tartaric acid and polyvalent phenols such astannic acid not only stabilize the preliminary chemical treatmentsolution but also act in complimenting the self-repair ability offluoride, and are therefore effective in improving adhesion.

An orthophosphate or polyphosphate of any of various metals may be addedto the preliminary chemical treatment solution. Accordingly, a solubleor poorly-soluble metal phosphate or a complex phosphate is included inthe preliminary chemical conversion film.

A soluble metal phosphate or complex phosphate is eluted from thepreliminary chemical conversion film in a defective portion thereof andreacts with the plating component (Al or the like) of the aluminum-basedalloy plated steel sheet that is the substrate steel, to therebyprecipitate a poorly-soluble phosphate. Accordingly, the self-repairability of titanium fluoride is complimented. Moreover, because themoisture in the atmosphere is slightly acidified when the poorly-solublephosphate is dissociated, the hydrolysis of titanium fluoride and thegeneration of poorly-soluble titanium oxide or hydroxide are promoted.

The metal of the soluble metal phosphate or complex phosphate may be analkali metal or an alkali earth metal; manganese; and/or the like. Thesoluble metal phosphate or complex phosphate may also be added to thepreliminary chemical treatment solution in the form of any of variousmetal phosphates or as a blend mixture of any of various metal salts andphosphoric acid, polyphosphoric acid or phosphate.

On the other hand, a poorly-soluble metal phosphate or complex phosphateimproves film strength by diffusing in the preliminary chemicalconversion film to eliminate film defects. The metal of thepoorly-soluble phosphate or complex phosphate may be Al, Ti, Zr, Hf, Znand/or the like. The poorly-soluble phosphate or complex phosphate maybe added to the preliminary chemical treatment solution in the form ofany of various metal phosphates or as a blend mixture of any of variousmetal salts and phosphate, polyphosphoric acid, or phosphate.

An organic wax such as a fluorine-based wax, a polyethylene-based wax,and a styrene-based wax; an inorganic lubricant such as silica,molybdenum sulfide, and talc; and/or the like, may be added to thepreliminary chemical treatment solution. The lubrication of thepreliminary chemical conversion film may be improved by adding theabove-described organic wax, inorganic lubricant, and/or the like. It isthought that a low melting point organic wax bleeds on the film surfaceand lubrication is achieved when the coating of the preliminary chemicaltreatment solution is dried. On the other hand, a high melting pointorganic wax or inorganic lubricant is dispersed in the film. However, itis thought that lubrication is achieved by exposing the outermostsurface of the film surface in an island-like distribution.

When performing elemental analysis such as X-ray fluorescence, ESCA orthe like, of a preliminary chemical conversion film obtained by drying acoating of the preliminary chemical treatment solution, theconcentrations of oxygen and fluorine present in the preliminarychemical conversion film are measured. The concentration ratio (atomicratio) of the two elements, F/O ratio, is preferably at least 1/100.Accordingly, corrosion of the obtained chemically treated steel sheet isprevented. Specifically, when the element concentration ratio F/O(atomic ratio) is at least 1/100, the occurrence of corrosionoriginating at the defective portion of the film is significantlydecreased. It is assumed that this was because a sufficient amount oftitanium fluoride was included in the preliminary chemical conversionfilm, and thus self-repair ability achieved.

Chemical Conversion Film

The chemical conversion film is formed on a surface of theabove-described aluminum-based-alloy plated steel (original sheet forchemical treatment). A preliminary chemical treatment such as formationof an underlying coating may or may not be performed on the surface ofthe original sheet for chemical treatment. Moreover, a chemicalconversion film is directly formed on the surface of the original sheetfor chemical treatment when no preliminary treatment is performed on thesurface of the original sheet for chemical treatment. The chemicalconversion film improves the weather resistance, the blackeningresistance and/or the like of the aluminum-based alloy plated steelsheet.

An object of the present invention is to improve the weather resistance,water resistance and film adhesion of the chemical conversion film. Aspreviously described, in order to improve the weather resistance(ultraviolet light resistance) of the chemical conversion film, afluororesin may be used as an organic resin. Fluororesins are broadlyclassified into solvent-based fluororesins and water-based fluororesins.Although recovery of an evaporated solvent is a problem when forming achemical conversion film using a solvent-based fluororesin, this problemdoes not occur when a water-based fluororesin is employed. The presentinventors attempted to form a chemical conversion film that was superiorin weather resistance, water resistance and film adhesion, by using awater-based fluororesin that is easy to handle.

As previously mentioned, according to preliminary experiments conductedby the present inventors, a reduction in water resistance of thechemical conversion film when it was formed using a water-basedfluororesin emulsion was thought to be because the emulsifier used whenpreparing the water-based fluororesin emulsion remained in the chemicalconversion film (see the below-mentioned reference experiments). Thepresent inventors thought that, if a water-based fluororesin emulsioncould be prepared using almost no emulsifier, a reduction in the waterresistance of the chemical conversion film could be prevented.Accordingly, as a result of an investigation into various water-basedfluororesins, the present inventors have found that a fluororesin havinga predetermined amount of hydrophilic functional groups introducedthereinto may be used to prepare a water-based emulsion using almost noemulsifier, and may easily form a chemical conversion film that includesalmost no emulsifier.

Moreover, the present inventors performed experiments not only onpreventing a reduction in the water resistance of the chemicalconversion film, but also on improving water resistance thereof. Inaddition, as a result of investigations performed from variousperspectives, it was found that the water resistance of the chemicalconversion film could be remarkably improved by increasing the molecularweight of the water-based fluororesin and crosslinking the water-basedfluororesin with a group 4A metal compound.

In addition, the present inventors found that a chemical conversion filmthat is superior in weather resistance, water resistance and filmadhesion can be formed by further formulating a group 4A metal compoundinto a chemical treatment solution containing as a primary component ahigh molecular weight fluororesin having a hydrophilic functional groupintroduced thereinto.

In the chemical conversion film of the chemically treated aluminum-basedplated steel sheet of the present invention, weather resistance(ultraviolet light resistance) is improved by 1) mixing a fluororesin(preferably a fluorine-containing olefin resin). Moreover, weatherresistance (ultraviolet light resistance) and water resistance areimproved by 2) decreasing the usage of an emulsifier during emulsionpreparation as much as possible by use of a fluororesin havinghydrophilic functional groups introduced thereinto; 3) increasing themolecular weight of the fluororesin; and 4) crosslinking the fluororesinwith a group 4A metal compound.

Hereinafter, each component included in the chemical conversion filmwill be explained.

1) Water-Based Fluororesin

The chemical conversion film contains a fluororesin as a primarycomponent, and more specifically a fluorine-containing olefin resin. Theamount of fluororesin contained in the chemical conversion film as theprimary component is preferably in a range of 70-99% by weight. Aspreviously described, weather resistance (ultraviolet light resistance)of the chemical conversion film may be improved by using a fluororesinas the organic resin that constitutes the chemical conversion film.

A water-based fluororesin that is easy to handle, is more preferablethan an organic solvent-based fluororesin. The term “water-basedfluororesin” refers to fluororesin having a hydrophilic functionalgroup. Examples of desirable hydrophilic functional groups may includecarboxyl group, sulfonic group, and salts thereof. Examples of salts ofcarboxyl group or sulfonic group may include ammonium salts, aminesalts, and alkali metal salts.

A desired water-based fluororesin (preferably a fluorine-containingolefin resin) contains 0.05-5% by weight of a hydrophilic functionalgroup. The fluororesin-containing 0.05-5% by weight of a hydrophilicfunctional group can be converted into a water-based emulsion even usingalmost no emulsifier. A chemical conversion film that contains almost noemulsifier can serve as a chemical conversion film that is superior inwater resistance.

The amount of a hydrophilic functional group in the water-basedfluororesin may be determined by dividing the total molar mass of thehydrophilic functional groups included in the water-based fluororesin bythe number-average molecular weight of the water-based fluororesin.Because the molar mass of carboxyl group is 45 and the molar mass ofsulfonic acid group is 81, the total molar mass of the hydrophilicfunctional groups included in the water-based fluororesin is found bycalculating the numbers of carboxyl groups and sulfonic acid groups,multiplying their molar mass by the respective numbers of carboxylgroups and sulfonic acid groups, and summing the products. Thenumber-average molecular weight of the water-based fluororesin ismeasured by GPC.

While the carboxyl group in the water-based fluororesin contributes toan improvement in the adhesion between the chemical conversion film andplating layer surface by forming a hydrogen bond with the plating layersurface, the crosslinking reaction with the group 4A metal compound isinhibited because H⁺ does not readily dissociate. On the other hand,although the sulfonic acid group in the water-based fluororesin readilydissociates H⁺, when the sulfonic acid group remains unreacted in thefilm without undergoing a cross-linking reaction with the group 4A metalcompound, the water resistance of the film may be remarkably reducedbecause of a strong adsorption effect of water molecules. Accordingly,both the carboxyl group and the sulfonic acid group are preferablyincluded in the water-based fluororesin, in order to make use of theirrespective properties. In such a case, the ratio of carboxyl groups tosulfonic acid groups in terms of carboxyl group/sulfonic acid groupmolar ratio is preferably within a range of 5-60.

The number-average molecular weight of the water-based fluororesinincluded in the chemical conversion film (preferably afluorine-containing olefin resin) is preferably at least 1,000, morepreferably at least 10,000, and even more preferably at least 200,000.

When the molecular weight of the water-based fluororesin included in thechemical conversion film is too low, the water permeability and waterresistance of the chemical conversion film cannot be sufficientlyimproved. In such a case, because moisture, corrosive gas, and/or thelike, easily passes through the chemical conversion film to reach theplated steel sheet, the plated steel sheet may be easily corroded.Moreover, because a radical generated by an effect of optical energyand/or the like easily acts on a terminal of a polymer chain when usinga water-based fluororesin with a low molecular weight, a water-basedfluororesin may be easily hydrolyzed by the synergistic effects of waterand/or the like. In order to prevent these problems, the molecularweight of the water-based fluororesin included in the chemicalconversion film may be increased to some extent and a cross-linkingstructure may be formed between chains of the water-based fluororesin.The water resistance is improved by increasing the molecular weight ofthe water-based fluororesin because van der Waals' force is strengthenedand therefore the density of the chemical conversion film is increased.Moreover, hydrolyzation is inhibited because the bonding between atomsin the backbone of the water-based fluororesin is stabilized.

On the other hand, the number-average molecular weight of thewater-based fluororesin included in the chemical conversion film ispreferably no more than 2,000,000. When the number-average molecularweight exceeds 2,000,000, problems may occur in chemical treatmentsolution stability such as gelation.

The fluorine atom content in the water-based fluororesin included in thechemical conversion film is preferably within a range of 7-20% byweight. When the fluorine atom content is less than 7% by weight, thewater resistance of the chemical conversion film cannot be sufficientlyimproved. On the other hand, when the fluorine atom content exceeds 20%by weight, coating is difficult to perform, and adhesion and dryingproperty may be reduced. The fluorine atom content of the water-basedfluororesin may be measured using an X-ray fluorescence analyzer.

A copolymer of a fluoroolefin and a hydrophilic functionalgroup-containing monomer is exemplified as the water-based fluororesin.The hydrophilic functional group-containing monomer is a carboxylgroup-containing monomer or a sulfonic acid group-containing monomer.

Examples of the fluoroolefin may include tetrafluoroethylene;trifluororethylene; chlorotrifluoroethylene; hexafluoropropylene; vinylfluoride; vinylidene fluoride; pentafluoropropylene;2,2,3,3-tetrafluoropropylene; 3,3,3-trifluoropropylene;bromotrifluoroethylene; 1-chloro-1,2-difluoroethylene; and1,1-dichloro-2,2-difluoroethylene. These fluoroolefins may be used aloneor in combination. From the perspective of weather resistance(ultraviolet light resistance), a perfluoroolefin such astetrafluoroethylene and hexafluoropropylene, or a vinylidene fluoride ispreferable among these fluoroolefins. A fluoroolefin including achlorine such as chlorotrifluoroethylene is not preferable becausecorrosion from chlorine ions may occur.

Unsaturated carboxylic acids such as those represented by formula (1)below and ester or acid anhydride thereof may be exemplified as examplesof a carboxyl group-containing monomer.

wherein, R¹, R² and R³ are the same or different, and R¹, R² and R³denote a hydrogen atom, an alkyl group, a carboxyl group, or an estergroup; and n is within a range of 0-20.

Examples of the unsaturated carboxylic acid having formula (1) mayinclude acrylic acid, methacrylic acid, vinyl acetate, crotonic acid,cinnamic acid, itaconic acid, itaconic acid monoester, maleic acid,maleic acid monoester, fumaric acid, fumaric acid monoester, 5-hexenoicacid, 5-heptenoic acid, 6-heptenoic acid, 7-octenoic acid, 8-nonenoicacid, 9-decenoic acid, 10-undecenoic acid, 11-decenoic acid,17-octadecenoic acid, and oleic acid.

A carboxyl group containing vinyl ether monomer having formula (2) belowmay be exemplified as an example of another carboxyl group-containingmonomer.

wherein, R⁴ and R⁵ are the same or different, and R⁴ and R⁵ denote asaturated or an unsaturated straight-chain or cyclic alkyl group; n is 0or 1; and m is 0 or 1.

Examples of the carboxyl group containing vinyl ether monomer havingformula (2) above may include 3-(2-aryloxyethoxycarbonyl)propionic acid;3-(2-aryloxybutoxycarbonyl)propionic acid;3-(2-vinyloxyethoxycarbonyl)propionic acid; and3-(2-vinyloxybutoxycarbonyl)propionic acid.

Examples of a sulfonic acid-containing monomer may include vinylsulfonicacid; allylsulfonic acid; methallyl sulfonic acid; styrenesulfonic acid;2-acrylamide-2-methylpropanesulfonic acid;2-methylacryloyloxyethanesulfonic acid; 3-methacryloyloxypropanesulfonicacid; 4-methacryloyloxybutanesulfonic acid;3-methacryloyloxy-2-hydroxypropanesulfonic acid;3-acryloyloxypropanesulfonic acid; allyloxybenzenesulfonic acid;methallyloxybenzenesulfonic acid; isoprenesulfonic acid; and3-aryloxy-2-hydroxypropanesulfonic acid.

Additional copolymerizable monomer(s) may be further copolymerized witha copolymer of fluoroolefin and hydrophilic functional group-containingmonomer, where necessary. A carboxylic acid vinylester, analkylvinylesters and a non-fluorine-based olefin may be exemplified asthe additional copolymerizable monomer.

The carboxylic acid vinyl ester allows compatibility and glossiness tobe improved, and the glass transition temperature to be increased.Examples of a carboxylic acid vinyl ester may include vinyl acetic acid,vinyl propionic acid, vinyl acetate, vinyl isoacetate, vinyl pivalate,vinyl caproate, vinyl versatate, vinyl laurate, vinyl stearate, vinylcyclohexanecarboxylic acid, vinyl benzoate, and vinylpara-t-butylbenzoate.

Alkylvinylesters allow glossiness and plasticity to be improved.Examples of an alkylvinylester may include methylvinylether,ethylvinylether, and butylvinylether.

Non-fluorine-based olefins allow flexibility to be improved. Examples ofa non-fluorine-based olefin may include ethylene, propylene, n-butene,and isobutene.

A fluoroolefin copolymer that has a hydrophilic functional group may beobtained by copolymerizing the above-described monomer with a knownpolymerization method. Accordingly, a water-based emulsion of afluoroolefin copolymer may be prepared using almost no emulsifier byadjusting the amount of fluoroolefin in the original monomer compositionso that the fluoroolefin copolymer includes 0.05-5% by weight of ahydrophilic functional group. Almost no emulsifier is included in achemical conversion film formed using an emulsion of the fluoroolefincopolymer containing almost no emulsifier (no more than 1% by weight).

Accordingly, chemical conversion film that contains almost no emulsifiercan be easily formed using, as a water-based fluororesin constitutingthe chemical conversion film, a fluororesin that has a hydrophilicgroup. A chemical conversion film formed in this manner achievessuperior water resistance with almost no degradation observed in waterresistance caused by a residual emulsifier.

2) Group 4A Metal Compound

The chemical conversion film includes a group 4A metal compound. Thegroup 4A metal compound readily reacts with a functional group such ascarboxyl group or sulfonic acid group in a water-based fluororesin, andpromotes the curing or cross-linking reaction of the water-basedfluororesin. Accordingly, the water resistance of the chemicalconversion film may be improved even by low temperature drying.

Weather resistance readily deteriorates when a melamine resin, anisocyanate resin or the like is used in the cross-linking of thefluororesin. For example, a chemical conversion film cured by means of amelamine resin rapidly undergoes weather resistance deterioration due tooxidation and/or hydrolysis of ester bonds, formether bonds and/or thelike. Moreover, the deterioration in weather resistance proceeds due tocleavage of the cross-linked structure by an acidic substance such assulfate ions and nitrate ions contained in acid rain. Because a urethanebond formed in a cross-linked portion is weaker than a fluorine bond inthe chemical conversion film cured using an isocyanate resin, thecross-linking structure is preferentially cut, and thus thedeterioration in weather resistance proceeds.

Such a problem can be avoided and water resistance can also be improvedby using a group 4A metal compound for the cross-linking of thefluororesin.

Moreover, the 4A metal compound improves the film adhesion and waterresistance. Specifically, although a hard aluminum oxide present on thesurface of the aluminum-based alloy plated steel sheet reduces theadhesion of the chemical conversion film, a reduction in film adhesionresulting from the aluminum oxide may be prevented by adding a group 4Ametal compound in the chemical conversion film. Moreover, the group 4Ametal compound also serves as an ion supply source of the group 4A metalions that react with aluminum ions eluted by an etching reaction. Areaction product is concentrated on the interface between the platinglayer and the chemical conversion film, to thereby allow initialcorrosion resistance to be improved. Examples of a group 4A metal mayinclude Ti, Zr, and Hf.

The amount of the group 4A metal compound of the chemical conversionfilm is preferably in a range of 0.1-5% by weight in terms of metal withrespect to the fluororesin. When the amount is less than 0.1% by weightin terms of metal, the adverse effects that are attributed to theconcentration of the aluminum oxide cannot be sufficiently prevented orthe water-based fluororesin cannot be sufficiently cross-linked, andthus the water resistance of the chemical conversion film cannot besufficiently improved. On the other hand, when the amount exceeds 5% byweight in terms of metal, the chemical conversion film may becomeporous, and workability and weather resistance may be reduced.

A metal equivalent amount of the group 4A metal compound in the chemicalconversion film can be measured using an X-ray fluorescence analyzer.

As previously described, aluminum is present which eluted from theplating layer in the chemical conversion film. This aluminum contributesto an improvement in corrosion resistance. The improvement in corrosionresistance based on the presence of aluminum is assumed to be the resultof the below-mentioned mechanism. Specifically, 1) because the chemicaltreatment solution is a weak alkaline, an aluminum oxide and metallicaluminum included in the plating layer are selectively eluted in thechemical treatment solution when the plating layer is coated with achemical treatment solution. 2) Aluminum elutes in the chemicaltreatment solution in the form of Al(OH)₄ ⁻ in a pH range of thechemical treatment solution. 3) The aluminum in the chemical treatmentsolution is incorporated in the chemical conversion film via adehydration-condensation reaction and/or the like when forming thechemical conversion film by drying the chemical treatment solution. 4)As a result thereof, the insulation property, density, and/or the like,of the chemical conversion film are improved, to thereby improvecorrosion resistance thereof.

3) Phosphate

The chemical conversion film preferably further contains a phosphate.The phosphate improves the adhesion to the aluminum-based alloy platedsteel sheet of the chemical conversion film by reacting with the platinglayer surface of the aluminum-based alloy plated steel sheet.

The type of phosphate is not specifically limited so long as thephosphate is a compound having a phosphate anion and the phosphate iswater-soluble. Examples of the phosphate may include sodium phosphate,ammonium phosphate, magnesium phosphate, calcium phosphate, manganesephosphate, zinc phosphate, orthophosphoric acid, metaphosphoric acid,pyrophosphoric acid (diphosphoric acid), triphosphoric acid, andtetraphosphoric acid. These phosphates may be used alone or incombination.

The amount of the phosphate in the chemical conversion film ispreferably within the range of 0.05-3% by weight as an amount in termsof phosphorus with respect to the fluororesin. When the amount in termsof phosphorus is less than 0.05% by weight, the reaction with theplating layer surface is incomplete, and thus the adhesion of thechemical conversion film cannot be sufficiently improved. On the otherhand, when the amount in terms of phosphorus exceeds 3% by weight, thereaction with the group 4A metal compound excessively proceeds, and thusa cross-linking effect resulting from the group 4A metal compound iscompromised.

The amount in terms of phosphorus of the phosphate in the chemicalconversion film may be measured by using an X-ray fluorescence analyzer.

4) Silane Coupling Agent

The chemical conversion film preferably further contains a silanecoupling agent. The adhesion of the chemical conversion film may befurther improved by the mixing of a silane coupling agent. A silanecompound including at least one functional group such as amino group,epoxy group, mercapto group, acryloxy group, methacryloxy group, alkoxygroup, vinyl group, styryl group, isocyanate group, and chloropropylgroup may be employed as the silane coupling agent.

The amount of the silane coupling agent in the chemical conversion filmis preferably within the range of 0.5-5% by weight with respect to thefluororesin. When the amount of the silane coupling agent is less than0.5% by weight, the adhesion of the chemical conversion film cannot besufficiently improved. On the other hand, when the amount of the silanecoupling agent exceeds 5% by weight, the film adhesion is saturated andno further improvement can be achieved.

The amount of silane coupling agent in the chemical conversion film maybe measured by using an X-ray fluorescence analyzer.

The thickness of the chemical conversion film is preferably with in therange of 0.5-10 μm. When the thickness is less than 0.5 μm, corrosionresistance, tarnish resistance, and/or the like, cannot be sufficientlyachieved. On the other hand, even when the thickness is set to greaterthan 10 μm, an increase in performance along with an increase inthickness cannot be expected.

2. Method of Preparing a Chemically Treated Aluminum-Based Plated SteelSheet

A method of preparing the chemically treated aluminum-based plated steelsheet of the present invention is not particularly limited in anymanner. For example, the chemically treated aluminum-based plated steelsheet of the present invention may be prepared by the below-mentionedmethod.

The method of preparation for the chemically treated aluminum-basedplated steel sheet of the present invention includes: 1) a first step ofpreparing an aluminum-based alloy plated steel sheet (original sheet forchemical treatment); 2) a second step of preparing a chemical treatmentsolution; and 3) a third step of forming the chemical conversion film onthe surface of the aluminum-containing aluminum-based alloy plated steelsheet. Moreover, when forming the chemical conversion film on thesurface of the aluminum-based alloy plated steel sheet with apreliminary chemical conversion film, the method of preparation for thechemically treated aluminum-based plated steel sheet further includes astep of forming the preliminary chemical conversion film by coating anddrying the preliminary chemical treatment solution on the surface of thealuminum-based alloy plated steel sheet before the third step in whichthe chemical conversion film is formed.

Preparation of Original Sheet for Chemical Treatment

In the first step, the above-described aluminum-based alloy plated steelsheet is prepared as the original sheet for chemical treatment.

Preparation of Chemical Treatment Solution

In the second step, the chemical treatment solution including thefluororesin (preferably a fluorine-containing olefin resin) having theabove-described hydrophilic functional group and the group 4A metalcompound is prepared.

The chemical treatment solution can be prepared by the addition of agroup 4A metal compound to the above-described water-based emulsion of afluororesin having a hydrophilic functional group (preferablyfluorine-containing olefin resin). A group 4A metal oxoate, fluoride,hydroxide, organic salt, carbonate, peroxygenated salt, and/or the like,may be used as the group 4A metal compound added to the chemicaltreatment solution. Examples of an oxoate may include hydrobromide,ammonium salt, alkaline metal salt, and alkaline earth metal salt. Aphosphate, a silane coupling agent and/or the like may also be added tothe chemical treatment solution where necessary.

The number-average molecular weight of the fluororesin included in thewater-based emulsion is preferably at least 1000, more preferably atleast 10,000, and even more preferably at least 200,000. As previouslydescribed, this is to impart water resistance in the chemical conversionfilm. On the other hand, from the perspective of treatment solutionstability, the number-average molecular weight of the fluororesin ispreferably no more than 2,000,000.

The fluororesin preferably contains 0.05-5% by weight of hydrophilicfunctional groups from the perspective of formulating a water-basedemulsion containing almost no emulsifier.

The amount of the emulsifier in the fluororesin water-based emulsion ispreferably no more than 1% by weight. When the emulsifier exceeds 1% byweight, the emulsifier may remain in the chemical conversion film,depending on the drying temperature, at the time that the chemicalconversion film is formed in the third step. When the emulsifier remainsin a chemical conversion film formed in this manner, it is notpreferable because the water resistance of the chemical conversion filmis remarkably reduced. As previously described, so long as thefluororesin contains a hydrophilic functional group, the water-basedemulsion can be formulated even when the amount of the emulsifier isless than 1% by weight.

As an emulsifier that may be included in a fluororesin water-basedemulsion, a fluorine-based emulsifier such as an ammonium salt ofperfluorooctanoic acid and an ammonium salt of perfluorononanoic acidare preferable from the perspective of weather resistance and waterresistance. In addition, any of the fluorine-based surfactants known inthe art may also be used as an emulsifier.

The amount of the fluororesin in the chemical treatment solution ispreferably within 10-70 parts by weight with respect 100 parts by weightof water. When the amount of the fluororesin is less than 10 parts byweight, there is a considerable amount of water evaporation in a dryingstep, and thus there may be a reduction in film formability and densityof the chemical conversion film. On the other hand, when the amount offluororesin included exceeds 70 parts by weight, the storage stabilityof the chemical treatment solution may be reduced.

The amount of oxoate, fluoride, hydroxide, organic salt, carbonate orperoxygenated salt of the group 4A metal in the chemical treatmentsolution is preferably with in the range of 0.1-5 parts by weight interms of metal with respect to 100 parts by weight of the fluororesin.When the amount of these salts is less than 0.1 parts by weight, across-linking reaction and a reaction with the plating layer surface isinhibited, and thus water resistance and film adhesion of the chemicalconversion film cannot be sufficiently improved. On the other hand, whenthe amount of these salts exceeds 5 parts by weight, the cross-linkingreaction proceeds, and thus the storage stability of the chemicaltreatment solution may be reduced.

When a phosphate is added to the chemical treatment solution, the amountof the phosphate in the chemical treatment solution is preferably 0.05-3parts by weight in terms phosphorus with respect to 100 parts by weightof the fluororesin. When the amount of the phosphate is less than 0.05parts by weight, the adhesion of the chemical conversion film cannot besufficiently improved. On the other hand, when the amount of thephosphate exceeds 3 parts by weight, the reaction with the group 4Ametal compound excessively proceeds, and thus the cross-linking effectresulting from the group 4A metal compound may be compromised.

When the silane coupling agent is added to the chemical treatmentsolution, the amount of the silane coupling agent in the chemicaltreatment solution is preferably within a range of 0.5-5 parts by weightwith respect to 100 parts by weight of the fluororesin. When the amountof the silane coupling agent is less than 0.5 parts by weight, theadhesion of the chemical conversion film cannot be improved. On theother hand, when the amount of the silane coupling agent exceeds 5 partsby weight, the film adhesion is saturated, and no further improvementcan be achieved. Moreover, the stability of the treatment solution maybe reduced.

An etching agent, an inorganic compound, a lubricant, a colored pigment,a dye and/or the like may be added where necessary to the chemicaltreatment solution as an additional component. A fluoride, and/or thelike, may be used as the etching agent. The etching agent furtherincreases the adhesion of the chemical conversion film via activation ofthe plating layer surface. Inorganic compound (e.g., oxides orphosphates) of Mg, Ca, Sr, V, W, Mn, B, Si, Sn and/or the like improvewater resistance by densifying the chemical conversion film. An organiclubricant such as a fluorine-based lubricant, a polyethylene-basedlubricant, or a styrene-based lubricant; or an inorganic lubricant suchas molybdenum sulfide or talc may be introduced into the film from thechemical treatment solution, or lubrication of the chemical conversionfilm, to further improve the workability of the chemically treatedaluminum-based alloy plated steel sheet. Moreover, a predetermined colortone may be achieved in the chemical conversion film by compounding aninorganic pigment, an organic pigment, an organic dye, and/or the like.

Formation of Chemical Conversion Film

In the third step, the chemical conversion film is formed on the surfaceof the aluminum-based alloy plated steel sheet prepared by the firststep. In forming the chemical conversion film, the chemical treatmentsolution prepared by the second step may be coated and dried on thesurface of the aluminum-based alloy plated steel sheet prepared by thefirst step.

A method of coating the chemical treatment solution is not particularlylimited, and thus may be appropriately selected from a well-knownmethod. Examples of such a coating method may include roll coatingmethod, curtain flow method, spin coating method, spray coating method,and dip coating method.

Although drying of the chemical treatment solution may be performed atroom temperature, drying time is preferably reduced by keeping thetemperature at no less than 50° C. when considering a continuousoperation. However, when the temperature is maintained at over 300° C.,the performance of the chemical conversion film may be reduced bythermal decomposition of the organic components. Because of the smallamount of emulsifier included in the chemical treatment solution used inthe method of preparation of the present invention, a chemicalconversion film that is superior in water resistance can be formedbecause almost no emulsifier is included even at a drying temperature ofaround 50° C.

According to the above-described method, the chemically treatedaluminum-based plated steel sheet of the present invention can beprepared which is superior in weather resistance, water resistance andfilm adhesion.

Formation of Preliminary Chemical Conversion Film

In a step in which the preliminary chemical conversion film is formed,the coating film is formed by coating the preliminary chemical treatmentsolution on the surface of the aluminum-based alloy plated steel sheetprepared by the first step before forming the chemical conversion film.The preliminary chemical treatment solution may be coated by, e.g., theroll coating method, spin coating method, or spray coating method. Thecoating amount of the preliminary chemical treatment solution ispreferably adjusted so that the deposition amount of a valve metal perunit area is at least 1 mg/m² in order that sufficient corrosionresistance is achieved in the obtained chemically treated steel sheet.Moreover, the coating amount of the preliminary chemical treatmentsolution is preferably formulated so that the thickness of the formedpreliminary chemical conversion film is 3 nm to no more than 1,000 nm.Sufficient corrosion resistance is exerted when the preliminary chemicalconversion film has a thickness of at least 3 nm. When the thicknessexceeds 1,000 nm, cracks may occur due to stress.

The preliminary chemical conversion film may be formed by drying withoutrinsing the coating film that has been formed on the surface of thealuminum-based alloy plated steel sheet. Although drying may also beperformed at room temperature, drying time is preferably reduced bykeeping the temperature at no less than 50° C. when considering acontinuous operation. However, when the drying temperature exceeds 200°C., the organic components included in the preliminary chemicalconversion film undergo thermal decomposition, and the propertiesimparted by the organic components are subsequently compromised.

Hereinafter, the present invention will be explained with reference toExamples, which however shall not be construed as limiting the scope ofthe invention thereto.

EXAMPLES Examples 1. Preparation of Chemically Treated Aluminum-BasedPlated Steel Sheet (1)

A hot-dip aluminum-9% by weight of silicon alloy plated steel sheet(coating weight: 45 g/m²) was fabricated using a cold-reduced carbonsteel sheet (SPCC) with a thickness of 0.8 mm as a substrate steel. Inthe present example, the hot-dip aluminum-9% by weight of silicon alloyplated steel sheet was used as an original sheet for chemical treatment.

Chemical conversion films with a thickness of 2.0 μm were formed bycoating a chemical treatment solutions of compositions shown in Table 1on the surface of the aluminum-based plated steel sheet, and performinghot-air drying at a peak sheet temperature of 140° C.

Chemical treatment solutions Nos. 1-10, which are shown in Table 1, wereprepared by adding group 4A metal compounds and/or the like, towater-based emulsions containing a fluororesin including a predeterminedamount of a carboxyl group and a sulfonic acid group, and an emulsifier(25% by weight of a non-volatile component; see Table 2). The chemicaltreatment solution No. 11 was prepared by adding a group 4A metalcompound, and/or the like, to a water-based emulsion containing aurethane resin and an emulsifier (25% by weight of a non-volatilecomponent; see Table 2).

A water-based emulsion including a fluororesin was obtained by adding apredetermined amount of a fluoroolefin, a carboxyl-containing monomer, asulfonic acid-containing monomer and an emulsifier to an aqueoussolvent, and copolymerizing these monomers. As a water-based emulsionincluding a urethane resin, PR135 is used (manufactured by Sumika BayerUrethane Co., Ltd.). As a silane coupling agent, A-1891 is used(manufactured by Momentive Performance Materials, Inc.).

TABLE 1 Compositions of Chemical Treatment Solutions Organic resin Group4A metal compound Other additives Concentration ConcentrationConcentration of organic in terms of a of additive resin in metal in inchemical chemical chemical Treatment treatment treatment treatmentsolution Emulsion No. solution solution solution no. (see Table 2) (g/L)Type (g/L) Type (g/L) 1 1 250 Hydrofluorotitanic acid Ti: 4 — — 2 2 250Fluorozirconic acid Zr: 3 — — 3 3 250 Ammonium zirconium Zr: 3 SilaneCoupling 10 carbonate agent 4 4 250 Oxalic acid titanium Ti: 1 SilaneCoupling 10 ammonium agent 5 5 250 Fluorozirconic acid Zr: 3 PhosphateP: 1 6 6 250 Fluorozirconic acid Zr: 3 Silane Coupling 10 agent 7 7 250Fluorozirconic acid Zr: 3 Silane Coupling agent 10 Phosphate P: 1 8 8250 Hydrofluorotitanic acid Ti: 4 — — 9 9 250 Ammonium zirconium Zr: 3 —— carbonate 10 10 250 — — Phosphate P: 1 11 11 250 Fluorozirconic acidZr: 3 Silane Coupling 10 agent

TABLE 2 Compositions of Organic Resin Emulsions Organic resin number-Amount of Fluorine atom Concentration Concentration average a functionalCarboxyl group/sulfonic amount in of organic resin of emulsifiermolecular group in resin acid group resin in emulsion in emulsionEmulsion No. Type weight (% by wt.) (molar ratio) (% by wt.) (g/L) (g/L)1 Fluorine- 10000 2.1 25 8 400 5 2 containing 10000 0.5 5 19 400 5 3olefin 10000 4.1 27 15 400 7 4 resin 10000 4.9 25 12 400 9 5 50000 4.935 16 400 4 6 100000 4.0 100 14 400 5 7 1000000 2.1 25 12 400 5 8Fluorine- 900 9.6 5 14 400 7 9 containing 10000 0.04 9 16 400 20 10olefin 10000 4.1 50 5 400 5 resin 11 Urethane 10000 4.1 50 0 400 8 resin

The amount of the group 4A metal, the amount of phosphate, the amount ofthe silane coupling agent in the chemical conversion film of eachchemically treated aluminum-based plated steel sheet was measured usingan X-ray fluorescence analyzer. The amount of the phosphate and theamount of the silane coupling agent were calculated from the measurementvalues for phosphorus and silicon. The amount of the group 4A metal, theamount of the phosphate, and the amount of the silane coupling agentwith respect to the organic resin in the formed chemical conversion filmfor the chemical treatment solutions are shown in Table 3.

TABLE 3 Compositions of Chemical conversion films Amount of group Amountof Amount of 4A metal with phosphate with silane coupling Treatmentrespect to respect to agent with respect solution organic resin organicresin to organic resin No. (% by wt.) (% by wt.) (% by wt.) 1 1.6 0 0 21.2 0 0 3 1.2 0 4 4 0.4 0 4 5 1.2 0.4 0 6 1.2 0 4 7 1.2 0.4 4 8 1.6 0 09 1.2 0 0 10 0 0.4 0 11 1.2 0 4

2. Evaluation of Chemically Treated Aluminum-Based Plated Steel Sheet(1)

(1) Accelerated Weather Resistance Test

A test piece was cut from each chemically treated aluminum-based platedsteel sheet and an accelerated weather resistance test (xenon lampmethod) was performed under the conditions regulated in JISK5600-7-7:2008. In the present test, as one cycle (two hours), water wassprayed to the test piece for 18-minutes within a 120 minute time periodin which light from a xenon arc lamp was irradiated, and this cycle wasrepeated for 0-1,000 cycles (0, 500, and 1,000 cycles).

(2) Evaluation of Weather Resistance

With regard to each chemically treated aluminum-based plated steelsheet, the thickness of the chemical conversion film before and afterthe accelerated weather resistance test was measured by cross-sectionalmicroscopic examination, and a coating film residual ratio wascalculated. Each of chemically treated aluminum-based plated steel sheetwas evaluated as “S” when the coating film residual ratio was at least95%, “A” when the coating film residual ratio was 80% to less than 95%,“B” when the coating film residual ratio was 60% to less than 80% “C”when the coating film residual ratio was 30% to less than 60%, and “D”when the coating film residual ratio was less than 30%.

(3) Evaluation of Film Adhesion

With regard to each chemically treated aluminum-based plated steelsheet, a film residual ratio of a portion in which the metal thereofmoved along a smooth surface with continuous contact on that surface wascalculated after performing a draw-bead test (bead height of metal: 4mm; welding force: 1.0 kN) using the test piece subjected or notsubjected accelerated weather resistance test (size: 30 mm×150 mm). Thefilm residual ratio was evaluated as “S” when the coating film residualratio was at least 95%, “A” when the coating film residual ratio was 80%to less than 95%, “B” when the coating film residual ratio was 60% toless than 80% “C” when the coating film residual ratio was 30% to lessthan 60%, and “D” when the coating film residual ratio was less than30%.

(4) Evaluation of Corrosion Resistance

With regard to each chemically treated aluminum-based plated steelsheet, a salt spray test (under the conditions regulated in JIS Z2371;120 hours) was performed using the test piece subjected to theaccelerated weather resistance test, and a white rust area ratio of aflat part was evaluated. Each of chemically treated aluminum-basedplated steel sheet was evaluated as “S” when the white rust area ratiowas no more than 5%, “A” when the white rust area ratio was exceeded 5%but was not more than 10%, “B” when the white rust area ratio exceeded10% but was not more than 30%, “C” when the white rust area ratioexceeded 30% but was not more than 50%, and “D” when the white rust arearatio exceeded 50%.

(5) Evaluation Results

With respect to each chemically treated aluminum-based plated steelsheet (Examples 1-7 and Comparative Examples 1-4), the type of treatmentsolution used, and the evaluation results of the weather resistancetest, the film adhesion test and the corrosion test of the flat part areall shown in Table 4.

TABLE 4 Corrosive Weather Film resistance Treatment resistance adhesionof flat part solution No. 0 500 1000 0 500 1000 0 500 1000 (see Table 1)cyc cyc cyc cyc cyc cyc cyc cyc cyc Example 1 1 S S S S S A S S AExample 2 2 S S S S S A S S A Example 3 3 S S S S S A S S S Example 4 4S S S S S A S S S Example 5 5 S S S S S A S S S Example 6 6 S S S S A AS A B Example 7 7 S S S S S S S S S Comp. Ex. 1 8 S B C S S B A B DComp. Ex. 2 9 S S C S S B B B D Comp. Ex. 3 10 S A C S B D C C D Comp.Ex. 4 11 S D D S D D S D D

The weather resistance was evaluated in terms of the coating filmresidual ratio of the chemical conversion film after the acceleratedweather resistance test. In the chemically treated aluminum-based platedsteel sheet of Comparative Example 4 including a chemical conversionfilm containing a urethane resin, the chemical conversion filmdisappeared after 500 cycles (equivalent to five years of outdoorexposure). On the other hand, in the chemically treated aluminum-basedplated steel sheets of Examples 1-7 including a chemical conversion filmcontaining a fluororesin having a predetermined amount of a hydrophilicgroup and a group 4A metal compound, the thickness of the chemicalconversion film hardly changed even after 1,000 cycles were repeated(equivalent to 10 years of outdoor exposure).

The film adhesion was evaluated based on the film residual ratio afterthe draw-bead test. Because the chemical conversion film disappearedafter 500 cycles (equivalent to five years of outdoor exposure) in thechemically treated aluminum-based plated steel sheet of ComparativeExample 4 including a chemical conversion film containing a urethaneresin, not even the film after the draw-bead test remained. On the otherhand, in the chemically treated aluminum-based plated steel sheets ofExamples 1-7 including a chemical conversion film containing afluororesin having a predetermined amount of a hydrophilic group and agroup 4A metal compound, the adhesion of the film was excellent evenafter 1000 cycles were repeated (equivalent to 10 years of outdoorexposure).

The corrosion resistance was further evaluated by the white rust arearatio after the salt spray test. In the chemically treatedaluminum-based plated steel sheet of Comparative Example 4 including achemical conversion film containing a urethane resin, while corrosionresistance was excellent before the accelerated weather resistance test,the corrosion resistance was remarkably reduced along with film loss.Moreover, in the chemically treated aluminum-based plated steel sheetsof Comparative Examples 1 and 2 including a chemical conversion filmcontaining a fluororesin having an excess amount or negligible amount ofa hydrophilic functional group and in the chemically treatedaluminum-based plated steel sheet of Comparative Example 3 including achemical conversion film that did not containing a group 4A metalcompound, the corrosion resistance was inferior even before theaccelerated weather resistance test. On the other hand, in thechemically treated aluminum-based plated steel sheets of Examples 1-7formed of a chemical conversion film containing a fluororesin having apredetermined amount of hydrophilic groups and a 4A group compound, thecorrosion resistance was excellent even after 1,000 cycles were repeated(equivalent to 10 years of outdoor exposure).

As seen from the above-described results, the chemically treatedaluminum-based plated steel sheet of the present invention is superiorin weather resistance, corrosion resistance, and film adhesion.

3. Preparation of Chemically Treated Aluminum-Based Plated Steel Sheet(2)

(1) Formation of Preliminary Chemical Conversion Film

The preliminary chemical conversion film was formed by hot-air drying ata peak sheet temperature of 70° C.-170° C., after coating thepreliminary chemical treatment solution of a composition indicated inTable 5 on a surface of the above-described aluminum-based alloy platedsteel sheet. Using an X-ray fluorescent analyzer, the composition of theformed preliminary chemical conversion film was analyzed. The resultsare shown in Table 6.

TABLE 5 Compositions of Preliminary Chemical treatment solutionsTreatment Valve metal source F source Organic acid Phosphate sourceSolution Valve metal Concentration of Concentration ConcentrationConcentration No. salt valve metal (g/L) Fluoride of F (g/L) Organicacid (g/L) Phosphate of P (g/L) 12 (NH₄)₂ZrF₆ 10 (Zr salt) 12.5 Tartaricacid 10 H₃PO₄ 6 13 Zr(SO₄)₂ 8 NH₄F 15 Tartaric acid 5 Mn(H₂PO₄)₂ 7.9 14Na₂WO₄ 3 (Ti salt) 14.4 Tannic acid 8 H₃PO₄ 30 (NH₄)₂TiF₆ 6 15 TiSO₄ 20(V salt) 15 Tannic acid 5 MgHPO₄ 12 VF₄ 10 16 K₂NbF₇ 16 (Nb salt) 22.9Oxalic acid 15 H₃PO₄ 20 17 K₂(MoO₂F₄) 20 (Mo 15.8 Tartaric acid 10(NH₄)H₂PO₄ 15 salt) 18 H₂TiF₆ 5 (Ti salt) 11.9 Citric acid 20 — — V₂O₅ 319 (NH₄)VO₃ 5 (Mo 3.9 Tartaric acid 5 — — Na₂(MoO₂F₄) 5 salt)

TABLE 6 Compositions of Preliminary Chemical Conversion Films Elementalconcentration Valve metal mass of preliminary chemical conversionTreatment of deposit film Solution per unit area (atomic %) No. (mg/m²)Valve metal O F P 12 Zr: 52 Zr: 17 76 4 3 13 Zr: 41 Zr: 15 74 6 5 14 W:15 W: 10 63 5 17 Ti: 30 Ti: 5 15 T: 44 Ti: 11 76 3 4 V: 21 V: 6 16 Nb:61 Nb: 15 75 4 6 17 Mo: 51 Mo: 19 75 2 4 18 Ti: 10 Ti: 11 72 10 0 V: 6V: 7 19 Mo: 21 Mo: 13 60 4 0 V: 20 V: 23

(2) Formation of Chemical Conversion Film

A chemical conversion film with a thickness of 2.0 μm was formed bycoating a chemical treatment solution with the above-described treatmentsolution No. 7 on the surface of the preliminary chemical conversionfilm, and performing hot-air drying at a peak sheet temperature of 140°C.

4. Evaluation of Chemically Treated Aluminum-Based Plated Steel Sheet(2)

An accelerated weather resistance test was implemented on the preparedchemically treated aluminum-based plated steel sheet by the same methodas the above-described “Evaluation of chemically treated aluminum-basedplated steel sheet (1).” Accordingly, weather resistance, blackeningresistance, and corrosion resistance of a flat part and crosscut portionof the prepared chemically treated aluminum-based plated steel sheetwere evaluated. The evaluation results are shown in Table 7. Thecorrosion resistance of the crosscut portion was evaluated according tothe below-indicated method.

Evaluation of Corrosion Resistance of Crosscut Portion

A test piece was obtained by forming an x-shaped cut-out portion(cross-cut portion) on the surface of each chemically treatedaluminum-based plated steel sheet and exposing the plating layer afterthe accelerated weather resistance test. A salt spray test was performedon the obtained test piece (under the conditions regulated in JIS Z2371;120 hours), and the white rust area ratio of the crosscut portion wasevaluated. Each of chemically treated aluminum-based plated steel sheetwas evaluated as “S” when the white rust area ratio was no more than 5%,“A” when the white rust area ratio was exceeded 5% but was not more than10%, “B” when the white rust area ratio exceeded 10% but was not morethan 30%, “C” when the white rust area ratio exceeded 30% but was notmore than 50%, and “D” when the white rust area ratio exceeded 50%.

TABLE 7 Treatment solution No. Preriminary Corrosive Corrosive chemicalChemical Weather Blackening resistance resistance of treatment treatmentresistance resistance of flat part crosscut part solution solution 0 5001000 0 500 1000 0 500 1000 0 500 1000 (see Table 5) (see Table 1) cyccyc cyc cyc cyc cyc cyc cyc cyc cyc cyc cyc Example 8 12 7 S S S S S S SS S S S S Example 9 13 7 S S S S S S S S S S S S Example 10 14 7 S S S SS S S S S S S S Example 11 15 7 S S S S S S S S S S S S Example 12 16 7S S S S S S S S S S S S Example 13 17 7 S S S S S S S S S S S S Example14 18 7 S S S S S S S S S S S A Example 15 19 7 S S S S S S S S S S S A

In the chemically treated aluminum-based plated steel sheet of Examples8-15 including a chemical conversion film containing a fluororesinhaving a predetermined amount of a hydrophilic group and a group 4Ametal compound on the preliminary chemical conversion film, there wasalmost no reduction in thickness and brightness of the chemicalconversion film even after 1,000 cycles were repeated (equivalent to 10years of outdoor exposure).

Moreover, favorable results were demonstrated in the corrosionresistance test of the flat part, even in all of Examples. Furthermore,very favorable results were demonstrated in the corrosion resistancetest of the crosscut portion, even in all of the Examples. It is thoughtthat this is because the self-repair ability of the preliminary chemicalconversion film has been achieved, and the crosscut portion (defectiveportion of the film) has been repaired. Particularly, in the chemicallytreated aluminum-based plated steel sheet of Examples 8-13 that includedphosphorus in the preliminary chemical conversion film, the corrosionresistance of the cross-cut portion was further improved.

Reference Test

The results of an investigation on a relationship between an amount of agroup 4A metal compound and an amount of emulsifier in a fluororesinfilm and permeability of the fluororesin film will now be described as areference test.

A fluororesin film with a thickness of 30 μm was formed by using a barcoater to coat a chemical treatment solution, which was formulated byadding a predetermined amount of a group 4A metal compound to afluororesin water-based emulsion having a hydrophilic functional group,which was prepared by adding an amount of a hydrophilic functional groupcontaining monomer so as to become 1% by weight and by adding an amountof an emulsifier so as to become 1% by weight, onto the surface of theplated steel sheet, and performing hot-air drying at a peak sheettemperature of 140° C. The fluororesin film was peeled from the platedsteel sheet, and cut into a predetermined size to be used as test piece.The permeability of each test piece (released fluororesin film) wasmeasured under the conditions regulated in JIS Z0208 (measurementconditions: temperature of 40±0.5° C., relative humidity 90±2%, and 24hour interval).

FIG. 1 is a graph showing a relationship between an amount of group 4Ametal in the fluororesin film and permeability of the fluororesin film.It is clear from the graph that the permeability of the fluororesin filmcan be remarkably reduced by using at least 0.1% by weight of a group 4Ametal amount in the fluororesin film.

A fluororesin film with a thickness of 30 μm was formed by using a barcoater to coat a chemical treatment solution, which was formulated byadding an amount so that a final concentration of a group 4A metalcompound in terms of metal would become 1% by weight, to a fluororesinwater-based emulsion having a hydrophilic functional group, which wasprepared by adding an amount of a hydrophilic functional groupcontaining monomer so as to become 1% by weight and by adding apredetermined amount of emulsifier, onto the surface of the plated steelsheet, and performing hot-air drying at a peak sheet temperature of 140°C. The fluororesin film was peeled from the plated steel sheet, and cutout into a predetermined size to be used as test piece. The permeabilityof each test piece (released fluororesin film) was measured under theconditions regulated in JIS Z0208 (measurement conditions: temperatureof 40±0.5° C., relative humidity 90±2%, and 24 hour interval).

FIG. 2 is a graph showing a relationship between a concentration ofemulsifier in a fluororesin water-based emulsion and a permeability ofthe fluororesin film. It is clear from the graph that the permeabilityof the fluororesin film can be remarkably reduced by using no more than1% by weight of emulsifier in the emulsion.

As seen from the above-described results, a fluororesin film containinga large quantity of a group 4A metal compound and small residualquantity of an emulsifier is superior in water resistance.

The disclosure of Japanese Patent Application No. 2010-139682, filed onJun. 18, 2010, and Japanese Patent Application No. 2011-042209, filed onFeb. 28, 2011, including the specification, drawings and abstract, areincorporated herein by reference in its entirety.

INDUSTRIAL APPLICABILITY

Because the chemically treated aluminum-based plated steel sheet of thepresent invention is superior in weather resistance, water resistanceand film adhesion, the chemically treated aluminum-based plated steelsheet of the present invention is useful in various applications such asan exterior building material. For example, the chemically treatedaluminum-based plated steel sheet of the present invention may besuitably used in an application such as 1) steel pipes, shaped steel,support posts, beams, and conveyance parts for a green house oragricultural house; 2) sound insulation walls, soundproof walls, soundabsorbing walls, snow barriers, guard rails, bridge railing, protectivefence, and supporting posts; and 3) railroad car parts, rail parts,parts for electric installation, parts for environmental safety,structural parts, and solar mounts.

The aluminum-based alloy plated steel sheet is sufficiently attached tothe coating film under a high temperature and high humidity, and issuperior in corrosion resistance. Accordingly, the chemically treatedaluminum-based plated steel sheet of the present invention isparticularly suitable as an exterior building material employed under ahigh temperature and high humidity environment.

The invention claimed is:
 1. A chemically treated aluminum-based platedsteel sheet, comprising: an aluminum-based alloy plated steel sheet, thealuminum-based alloy containing 85-99% by weight of aluminum; and achemical conversion film having a film thickness of 0.5-10 μm formed ona surface of the aluminum-based alloy plated steel sheet, wherein thechemical conversion film contains a fluororesin and a group 4A metalcompound, the fluororesin containing a total of 0.05-5% by weight of ahydrophilic functional group selected from the group consisting of acarboxyl group, a sulfonic acid group and a salt thereof, and 7-20% byweight of a fluorine atom, and the group 4A metal compound content, withrespect to the fluororesin, is within a range of 0.1-5% by weight interms of a group 4A metal of the group 4A metal compound.
 2. Thechemically treated aluminum-based plated steel sheet according to claim1, wherein a ratio between the carboxyl group and the sulfonic acidgroup included in the fluororesin in terms of a carboxyl group/sulfonicacid group molar ratio is 5-60.
 3. The chemically treated aluminum-basedplated steel sheet according to claim 1, wherein the chemical conversionfilm further contains a phosphate, and an amount of the phosphate withrespect to the fluororesin is within a range of 0.05-3% by weight interms of phosphorus.
 4. The chemically treated aluminum-based platedsteel sheet according to claim 1, wherein the chemical conversion filmfurther contains a silane coupling agent, and an amount of the silanecoupling agent with respect to the fluororesin is within a range of0.5-5% by weight.
 5. The chemically treated aluminum-based plated steelsheet according to claim 1, wherein the group 4A metal is selected fromthe group consisting of Ti, Zr, Hf, and any combination thereof.
 6. Thechemically treated aluminum-based plated steel sheet according to claim1, further comprising a preliminary chemical conversion film containingan oxide or hydroxide of a valve metal and fluoride of a valve metal,the preliminary chemical conversion film formed between thealuminum-based alloy plated steel sheet and the chemical conversionfilm.
 7. A method of preparing a chemically treated aluminum-basedplated steel sheet, the method comprising: preparing an aluminum-basedalloy plated steel sheet, the aluminum-based alloy including 85-99% byweight of aluminum; and forming a chemical conversion film that has athickness 0.5-10 μm, by coating and drying a chemical treatment solutionon a surface of the aluminum-based alloy plated steel sheet, wherein:the chemical treatment solution contains a fluororesin and at least oneselected from the group consisting of an oxoate, a fluoride, ahydroxide, an organic salt, a carbonate and a peroxygenated salt of agroup 4A metal, the fluororesin containing a total of 0.05-5% by weightof a hydrophilic functional group selected from the group consisting ofa carboxyl group, a sulfonic acid group and a salt thereof, and 7-20% byweight of a fluorine atom, the fluororesin having a number-averagemolecular weight of 1,000-2,000,000; and the total amount of the oxoate,the fluoride, the hydroxide, the organic salt, the carbonate and theperoxygenated salt of the group 4A metal is within a range of 0.1-5% byweight, in terms of the group 4A metal, with respect to the fluororesin.8. The method of preparing the chemically treated aluminum-based platedsteel sheet according to claim 7, wherein a ratio between the carboxylgroup and the sulfonic acid group included in the fluororesin in termsof a carboxyl group/sulfonic acid group molar ratio is 5-60.
 9. Themethod of preparing the chemically treated aluminum-based plated steelsheet according to claim 7, wherein the chemical treatment solutionfurther contains a phosphate, and an amount of the phosphate withrespect to the fluororesin is within a range of 0.05-3% by weight interms of phosphorus.
 10. The method of preparing the chemically treatedaluminum-based plated steel sheet according to claim 7, wherein thechemical treatment solution further contains a silane coupling agent,and an amount of the silane coupling agent with respect to thefluororesin is within a range of 0.5-5% by weight.
 11. The method ofpreparing the chemically treated aluminum-based plated steel sheetaccording claim 7, wherein the group 4A metal compound is selected fromthe group consisting of Ti, Zr, Hf, and any combination thereof.
 12. Themethod of preparing the chemically treated aluminum-based plated steelsheet according to claim 7, further comprising: forming a preliminarychemical conversion film by coating and drying a preliminary chemicaltreatment solution on a surface of the aluminum-based alloy plated steelsheet before forming the chemical conversion film, wherein thepreliminary chemical treatment solution contains a valve metal salt anda fluoride ion.